1. Field of Invention
The present invention is directed to methods for selectively reacting halocarbon isomers and to methods of selectively removing one or more halocarbon isomers from blends containing multiple isomeric forms. More particularly, the invention is directed to methods for the selectively reducing a halocarbon blend comprising 1,3-dichloro-1,1,2,2,3-pentafluoropropane. More specifically, the invention relates to methods for preparing hydrofluorocarbons via the selective reduction of a halocarbon blend comprising 1,3-dichloro-1,1,2,2,3-pentafluoropropane.
2. Description of Related Art
Many halocarbons, particularly lower halocarbons, find use in a variety of applications, including refrigerants, propellant gases, fire-extinguishing agents, blowing agents for foams, and many others. As used herein, the term “halocarbon” means a compound containing carbon, one or more halogens, and optionally hydrogen. Halocarbons of particular interest with respect to the present invention are “C3 halocarbons,” that is, halocarbons having three (3) carbon atoms in the chain, including C3 hydrochlorofluorocarbons, C3 hydrofluorocarbons, and C3 hydrofluoroolefins. Exemplary of such compounds are CF3CF2CHCl2 (HCFC-225ca), CClF2CF2CHClF (HCFC-225cb), CF3CF2CH2Cl (HCFC-235ca), CF3CF2CH3 (HFC-245cb), CF3CFHCH3 (HFC-254 eb), and CF3CF═CH2 (HFO-1234yf).
Applicants have also come to appreciate the need safe and effective starting materials for the production of certain C3 halocarbons, particularly C3 fluoroolefins. For example, applicants have come to appreciate the advantage of providing inexpensive feedstock for the production of relatively non-toxic, environmentally acceptable, and useful C3 halocarbons, such as trifluoropropenes (e.g., CF3CH═CH2 (HFC-1243zf)), tetrafluoropropenes (e.g., CF3CF═CH2 (HFC-1234yf)), pentafluoropropene (e.g., CF3CF═CFH (HFC 1225 ye)), and the like. Other C3 halocarbons which can be produced include HCF2CF2CH3, CF3CF2CH2Cl (HCFC 235ca), CF3CF2CH3 (HFC-245cb) and the like.
HCFC-225ca and HCFC-225cb are two isomers of dichloropentafluoropropane that have been proposed as alternatives to trichlorotrifluoroethane (CFC-113) for use as a cleaning solvent. CFC-113 is undesirable for use in many applications because it is considered to be environmentally unfriendly. HCFC-225ca and HCFC-225cb are known to be easily and economically obtained by the reaction of dichlorofluoromethane and tetrafluoroethylene. Blends of these HCFC-225 isomers have been commercialized. For example, Asahi Glass Co. of Japan markets a 47/53 weight percent blend of HCFC-225ca/cb under the name Asahiklin AK-225.
Applicant's have come to recognize the existence of toxicity studies indicating that both HCFC-225ca and HCFC-225cb have very low acute toxicity compared to CFC-113. [See e.g. PAFT HCFC-225ca/cb Testing Web Site (www.afeas.org/paft/hcfc-225.html) showing that CFC-113 has a cardiac sensitization response three times that of HCFC-225ca/HCFC-225cb blends]. Even though both of these dichlorotetrafluoropropane isomers have a relatively low level of toxicity, applicant's have further recognized that HCFC-225ca nevertheless has been found to be significantly more toxic than HCFC-225cb. For example, PAFT testing has shown that an HCFC-225ca exposure level of 650 to 1000 ppm had a substantially greater effect on the liver of the animal in comparison to HCFC-225cb, which at even greater exposure levels of 1000 to 5000 ppm had only marginal effects. Moreover, the US Environmental Protection Agency has set an average workplace standard exposure level of 250 ppm for the cb isomer, but only 25 ppm for the ca isomer. (See e.g. US EPA “Substitutes in Precision Cleaning” at www.epa.gov/ozone/snap/solvents/lists/precisio.html.)
In general, it is preferable to minimize workplace exposure to toxic chemicals. In a mixture or blend of compounds that may potentially be exposed to humans or other animals, for example during use, transportation and the like, it is generally therefore less of a disadvantage from a toxicity standpoint for the blend or mixture to contain HCFC-225cb than for it to contain a like amount of HCFC-225ca.
Reactions involving the conversion of chlorofluoropropanes, such as HCFC-225ca and/or HCFC-225cb, are known. For example, U.S. Pat. No. 5,663,543 (Morikawa) describes the oxidation of at least one dichloropentafluoropropane selected from HCFC-225ca and HCFC-225cb by oxygen under irradiation with and in the presence of chlorine. The product resulting from this reaction is a polyfluoropropionyl halide. Another example of such conversion can be found in U.S. Pat. No. 5,532,418 (Nakada) which teaches a method for producing a hexafluoropropane via a multi-step process whereby HCFC-225ca and/or HCFC-225cb is de-chlorofluorinated by hydrogen in the presence of a metal oxide catalyst to obtain a tetrafluorochloropropene, which is subsequently fluorinated to produce hexafluoropropane.
However, neither Morikawa nor Nakada teach or even suggest the selective conversion of HCFC-225ca over HCFC-225cb. In fact, Nakada actually teaches that under the reaction conditions specified therein HCFC-225cb is more readily converted to other compounds than HCFC-225ca. In addition, neither of these patents teach or suggest direct synthesis of a hydrofluorocarbon or hydrofluoroolefin from a dichloropentafluoropropane or blend of dichloropentafluoropropane isomers